This invention relates to an oxygen sensor for sensing the oxygen ion content of the exhaust gas from an internal combustion engine to control the air/fuel ratio in the combustion chamber. Especially, it relates to the oxygen sensor that uses an oxygen-conductive solid electrolyte material.
Oxygen sensors are used in an automobile engine to control the air/fuel ratio in the combustion chamber around the stoichiometric value, so that the fuel consumption is improved and the exhaust gas is purified. A known oxygen sensor includes an oxygen sensing element having an ion-conductive solid electrolyte plate on which porous electrodes are printed. The sensing element generates a voltage corresponding to the difference in oxygen content between the object gas (exhaust gas) and the reference air.
A recent trend of engine control is to bring the air/fuel ratio not only to the stoichiometric value but to any value according to the operating conditions of the engine to obtain better engine performance as well as to further improve the fuel efficiency and emission. Oxygen sensors appropriate to this need of variable air/fuel ratio control have been developed as, for example, in Japanese Published Unexamined Patent Application Nos. S52-72286 and S53-66292. In these sensors, a gas diffusion chamber is formed facing one of the electrodes of a solid electrolyte plate, and a certain voltage is applied between the electrodes to introduce the object gas into the chamber by diffusion process. The electrical current between the electrodes at that time shows the oxygen content of the object gas.
Another type of oxygen sensor is disclosed in Japanese Published Unexamined Patent Application No. S60-36032 where the oxygen sensor includes an oxygen pump element and an oxygen concentration cell element which are placed parallel to each other forming the gas diffusion chamber between them. Electric current to the oxygen pump element is controlled to keep the voltage between the electrodes of the oxygen concentration cell element constant, where the amount of current shows the oxygen content.
One big problem about the conventional oxygen sensors is that it takes a long time until the sensor becomes active after the engine operation starts. During the period (warm-up period), the engine air/fuel ratio feedback control is impossible. For this reason, recent oxygen sensors are equipped with heaters. But, the heating speed should be limited to a rather lower degree because too rapid heating causes thermal shock on the oxygen sensor which may damage the sensor element.